Diffuse Axonal Injury

This is Dr. Cal Shipley with a review of diffuse axonal injury. Diffuse axonal injury is a potentially severe form of traumatic brain injury and results from sudden changes in velocity of the head. The brain motion underlying diffuse axonal injury is rotational, also referred to as angular, as depicted here.

Diffuse Axonal Injury Physics

Taking a look at some of the physics involved in diffuse axonal injury, the brain is composed of both gray and white matter. These two types of tissue differ in their densities. Much of the gray matter is located on the surface of the cerebral hemispheres and the cerebellum. White matter forms the bulk of the deep portions of the cerebral hemispheres in the cerebellum, but there are aggregates of gray matters, such as the basal ganglia and brainstem nuclei scattered throughout. It is at the interface between the gray matter and white matter that most axonal injury occurs with rapid deceleration injuries of the brain.

Anatomy of the Neuron

Now let’s take a look at the anatomy of the neuron. The human brain is a complex web of billions of interconnected cells known as neurons. Neurons are specialized cells which have a nucleus. Each neuron cell has multiple arm-like projections called dendrites. Neurons also gives rise to a long filamentous structure known as an axon. Neurons can have multiple dendrites but usually have only one axon.

Neurons communicate with one another by sending chemically-generated electrical impulses through their dendrites and axons. The electrochemical impulses are transferred from one neuron to another at junctions known as synapses. Typically, the synapse occurs between the axon of one neuron cell and the dendrite of another. The synaptic junctions may also occur between dendrites or between axons. The axons are surrounded at intervals by tissue known as myelin, which facilitates movement of the electrical impulse and prevents leakage of the charge into surrounding tissues.

Normal myelination is critical to nervous system function and loss of myelin can lead to serious disorders such as multiple sclerosis. Tens of millions of neuronal axons lie across the junction of the gray matter and the white matter of the brain, and it is this orientation which gives rise to the nerve shear damage seen in diffuse axonal injury.

Angular Rotation and Diffuse Axonal Injury

As mentioned previously, in acceleration-deceleration injuries of the brain, it is the rotational or angular movement of the brain that causes diffuse axonal injury.

The gray matter and white matter have different densities. As a result, when the brain is accelerated, and then suddenly decelerated, in an angular motion, the gray matter and white matter move at different velocities. At their junction, the two tissues slide over one another stretching neuronal axons. This is known as shear effect. Stretching of the axons is known as primary injury, and this stretching, rather than actual tearing of the axons, during shear is considered to be the basis for diffuse axonal injury.

Secondary Neuronal Injury

Within hours to days after stretching of the nerve axons during primary injury, biochemical changes at the cellular level result in loss of axonal function. This is known as secondary injury, and it appears to occur in two forms. The first form of secondary injury involves swelling and rupture of the nerve axons with formation of characteristic axonal bulbs on the ruptured ends. This results in an immediate loss of nerve impulse transmission across the axon.

The second form of secondary injury is the formation of swellings, also known as varicosities, along the length of the axon. Axonal varicosities may result in only a partial loss of nerve impulse transmission.

Axonal Varicosities and Loss of Myelin

Loss of myelin may also occur in association with the formation of axonal varicosities, eventually resulting in complete loss of nerve impulse transmission. Some researchers believe that this demyelination process may take up to 24 months to occur after primary injury. How clinical symptoms correlate with this prolonged demyelination process is not currently well understood.

Please see my article on diffuse axonal injury for much more information on this topic.

Cal Shipley, M.D. copyright 2020